Chronic infection with HBV is a major risk factor in the development of hepatocellular carcinoma (HCC), and is responsible for over 1 million deaths each year. We have established an HCC cofactor role in transgenic mice that express the HBV regulatory protein (HBx) driven by the human alpha-l-anti-trypsin regulatory sequences (ATX mice). Our recent studies indicate that the cofactor role involves the ability of HBx to deregulate hepatocyte cell cycle controls. Although HBx is essential for virus replication in vivo, the function that it provides is not known. We hypothesize that the essential role of HBx during viral replication is to prepare hepatocytes to become competent to support all steps in virus replication. In the present application, we will explore the effects of HBx on G0 hepatocytes and their transition through early G1, using markers that distinguish these stages of the cell cycle. We additionally hypothesize that HBx holds nondividing cells at a G1 restriction point, and will use RNA inhibition to examine candidate proteins through which HBx may exert this effect. We will also explore the effect of HBx on hepatocyte G0/G1 transition in vivo, and in the presence of the AFBl-induced hotspot p53 mutation. Finally, the livers of ATX mice contain decreased levels of Caspase-3, and we will test the hypothesis that in response to cytotoxic cytokines, HBx inhibits apoptosis as a means of protecting hepatocytes from immune-mediated cell death. HBx is the sole regulatory protein encoded by HBV, we expect it will provide more than one critical role during the virus life cycle. The proposed program builds on our model of HBx-related HCC that indicates an important contribution of HBx effects on cell cycle control. The planned studies examine HBx function in nondividing liver cells, and the results obtained will provide important insight into the rational development of new therapies to treat chronic HBV infection. Our results will also reveal fundamental virus-host interactions that may extend to other viruses that replicate in the liver.